NMR logging techniques are sometimes used in estimating formation porosity and associated characteristics. NMR logging measures the induced magnet moment of hydrogen nuclei contained within the fluid-filled pore space of the formation. NMR logs provide information about the quantities of fluids present, the properties of these fluids, and the sizes of the pores containing these fluids.
NMR logging has the advantage of measuring only the response of the presence of hydrogen protons contained in oil, water and gas without interference from the matrix minerals. NMR logs can provide the information about the fluids and the interaction between the fluids and rock. The T2 spectrum, inverted from the time domain relaxation time profile, is a reflection of the pore size distribution which makes it possible to differentiate clay-bound water (CBW), bulk volume of the irreducible (BVI) water and free water index (FFI) for accurate estimation of recoverable reserves and to infer the permeability using Coates or Schlumberger models.
In NMR, T2 is a time constant which characterizes transverse relaxation of an NMR signal. T2 cutoff is a parameter used in interpretation of the NMR logging to distinguish the bulk volume of irreducible water (BVI) and free fluids (FFI). The BVI and FFI are used in calculation of recoverable reserve and, in the free fluid model, for estimating permeability.